The present invention relates to a catalytic combustion heater that heats an object fluid with heat from an oxidation reaction of a fuel gas with a catalyst, and, more particularly, to a catalytic combustion heater that has a short activation time, when the heater is activated.
A so-called catalytic combustion heater, which causes an oxidation reaction of a flammable gas (fuel gas) with a catalyst and heats an object fluid using the generated heat, is known, and various applications of the heater, such as home use and vehicular use, have been studied (for example, Japanese Unexamined Patent Publication (KOKAI) No. Hei 5-223201, etc.). A catalytic combustion heater is equipped with a catalyst-carrying heat exchanger having tubes located in a flow passage of a flammable gas and in which an object fluid to be heated which is a liquid or gas flows, with multiple catalyst-carrying fins integrally joined to the outer surfaces of the tubes. The multiple fins carry an oxidation catalyst, such as platinum or palladium. When the catalyst-carrying fins are heated to or above an activation temperature and contact a flammable gas, an oxidation reaction occurs on the surfaces of the fins. The oxidation reaction generates heat, which is transferred from the fins into the tubes to heat the object fluid that flows in the tubes.
The flammable gas is mixed with a combustion support gas (normally, air) for oxidizing the flammable gas, and is then supplied as a fuel gas into the catalyst-carrying heat exchanger. Because the catalyst-oriented oxidation reaction occurs in a very wide range of the flammable gas concentration, unburned gas, which has not reacted at an upstream location can be burned with a catalyst at a downstream location, and combustion can be carried out along the entire heat exchanger. This provides a compact and high-performance heater as compared with burner type heaters, which have been typical so far.
It is desirable to rapidly raise the temperature of fins to quickly make the catalyst of the entire system active when the catalytic combustion heater is activated. Normally, therefore, means for detecting multiple temperatures, such as the fin temperature, the temperature of the object fluid to be heated and the combustion-exhaust-gas temperature, based on a previously prepared map are provided, so that the flow rate of the object fluid is gradually increased to a specified rate while monitoring the temperatures. In the case of heating water of normal temperature to vapor at 300xc2x0 C., for example, the flow rate of the object fluid is controlled such that the flow rate of the object fluid is set to zero until the fin temperature on the upstream side of a flammable-gas flow passage reaches the activation temperature, and the fin temperature does not fall below the activation temperature thereafter while making sure that the other catalysts become active in order and the activation temperature is maintained.
However, the conventional catalytic combustion heater must monitor multiple temperatures or detect the fin temperature and the temperature of the object fluid at plural locations in the flammable-gas flow passage and requires complicated control procedure. Further, the heater may not start as expected, depending on variations in the initial temperature of the object fluid or the combustion support gas. Furthermore, if the flow rate of the object fluid is not controlled properly, e.g., if the flow rate is too small, the heat generated on the fin surfaces is not transferred away, which heats the fins and tubes locally and deteriorates the catalyst. If the flow rate is too large, on the other hand, the fin temperature is too low and the catalyst reaction does not occur. This leads to the discharge of unburned gas, which deteriorates the exhaust emissions. There is another problem in that it takes too long to activate the heat exchangers.
The present invention has been devised to overcome the above conventional problems, and it is an object of the present invention to provide a safe and quick-activating catalytic combustion heater that activates quickly with a simple structure while preventing local heating of the fins and the tubes and preventing discharge of unburned gas.
A catalytic combustion heater according to the present invention is equipped with a catalyst-carrying heat exchanger having tubes located in a fuel-gas flow passage, the interiors of which serve as an object fluid flow passage. Fins are joined to outer surfaces of the tubes and carry an oxidation catalyst for causing an oxidation reaction when contacting a fuel gas. An object fluid is heated by oxidation reaction heat of the fuel gas. The heater includes means for detecting the temperature of the object fluid in the vicinity of an outlet of the object fluid flow passage and flow-rate control means for controlling the flow rate of the object fluid when the heater is activated based on the temperature of the object fluid detected by the temperature detecting means. The flow-rate control means maintains a small flow rate of the object fluid until the temperature of the object fluid exceeds a predetermined temperature and increases the flow rate of the object fluid when the temperature of the object fluid exceeds the predetermined temperature.
When heating the object fluid, the amount of heat required to raise the liquid to the boiling point is smaller than the latent heat for converting the liquid to a gas. The way heat is transferred in the tubes varies in accordance with the state of the object fluid. For example, the heat transfer coefficient of an object fluid in a liquid state is lower than that of an object fluid in a boiling state, which is a gas-liquid mixed state. Accordingly, the activation control can be carried out well by detecting the temperature of the object fluid in the vicinity of the outlet of the passage where the object fluid becomes hottest, to find the state of the object fluid, and by controlling the flow rate of the object fluid based on that state. That is, in the initial stage of heating, the flow rate of the object fluid is maintained low to suppress heat transfer to the object fluid, thereby quickly raising the temperatures of the fins and tubes to the activation temperature. When the temperature of the object fluid exceeds a predetermined temperature, e.g., the boiling point, the flow rate of the object fluid is increased to increase the flow speed, which increases the heat transfer to the object fluid. This prevents the temperatures of the fins and tubes from becoming too high. In this manner, the generated heat can be used effectively to make the entire heater activate quickly. Therefore, the heater can provide the desired high temperature gas in a short activation time, has a simple structure, need not monitor multiple temperatures, and is very safe.
In one embodiment, the flow-rate control means performs control to set the flow rate of the object fluid to a low rate, when the heater is activated, so that the flow of the object fluid becomes laminar, maintains that flow rate until a typical boiling point of the object fluid is exceeded, and increases the flow rate of the object fluid to a specified rate when the temperature of the object fluid exceeds the typical boiling point.
Specifically, the flow rate of the object fluid is controlled based on the boiling point of the temperature of the object fluid, and the flow rate of the object fluid is kept low to make the flow speed sufficiently low. Particularly, if the flow of the object fluid is kept laminar, the heat resistance is increased, making heat transfer in the tubes difficult. Accordingly, the temperatures of the fins and tubes increase, thus ensuring quick activation. Since the quantity of the object fluid is small, it boils relatively quickly. Because the heat resistance abruptly decreases in the boiling state, and heat transfer becomes easier, the vaporization of the object fluid is increased while the flow rate is small. When all of the object fluid is vaporized, the heat transfer coefficient becomes low again, and, when the temperature of the object fluid exceeds the boiling point, the flow rate of the object fluid is increased at once. This increases the flow speed to increase the heat transfer to the object fluid, so that good activation control is performed in a short time while preventing the temperatures of the fins and tubes from becoming abnormally high.
In one embodiment, the flow-rate control means controls the flow rate of a combustion support gas while being mixed in the fuel gas, based on the temperature of the object fluid. Since the flow rate of the combustion support gas is controlled in addition to the control of the flow rate of the object fluid, the generated heat can be used more effectively.
In one embodiment, the direction of the flow of the fuel gas in the catalyst-carrying heat exchanger is opposite to the direction of the flow of the object fluid. At this time, the flow-rate control means performs control to increase the flow rate of the combustion support gas to or above a specified rate when the temperature of the object fluid exceeds its typical boiling point. In another embodiment, the control means performs control to decrease the flow rate of the combustion support gas to the specified rate when the temperature of the object fluid becomes stable in the vicinity of a target temperature.
When the direction of the flow of the fuel gas is opposite to that of the object fluid, the flow rate of the combustion support gas is not increased more than necessary and the flow speed of the flammable gas that contacts the fin surfaces is slowed until the object fluid in the vicinity of the outlet of the object fluid, where the fuel gas having a high flammable-gas concentration is supplied is boiled. This makes it hard to transfer the generated heat to the flammable gas, and the temperature of the catalyst quickly rises to the activation temperature. When the flow rate of the combustion support gas is increased, the transfer of the heat generated by the oxidation reaction becomes easier, the heat is carried downstream with the faster flowing fuel gas and the combustion exhaust gas. When the object fluid reaches the boiling point, where the heat resistance becomes low, the flow rate of the combustion support gas is increased to allow the downstream fins and tubes to be exposed to the high-temperature gas, so that the temperature of the entire heater is quickly increased to or above the catalyst activation temperature. When the temperature of the object fluid becomes stable in the vicinity of a predetermined temperature, the flow rate of the combustion support gas is decreased to the specified rate to reduce the amount of heat discharged with the combustion exhaust gas, so that the heat exchanging efficiency is well maintained.
In one embodiment, the direction of the flow of the fuel gas in the catalyst-carrying heat exchanger is the same as the direction of the flow of the object fluid. Further, the flow-rate control means performs control to make the flow rate of the combustion support gas greater than a specified rate from when the heater is activated until the temperature of the object fluid exceeds its typical boiling point and to decrease the flow rate of the combustion support gas to the specified rate when the temperature of the object fluid exceeds the typical boiling point.
When the direction of the flow of the fuel gas is the same as that of the object fluid, it is better to quickly make the catalyst active on the downstream side of the fuel-gas flow passage, where the temperature of the object fluid is the highest, to prevent deterioration of the exhaust emissions. Therefore, the heat generated on the fin surfaces is more easily transferred to the flammable gas by making the flow rate of the combustion support gas greater than the specified rate from the time when the heater is activated until the temperature of the object fluid in the vicinity of the object fluid outlet boils. This causes the downstream fins and tubes to be exposed to the high-temperature gas, thus ensuring a quick increase in the catalyst activation temperature. When the object fluid exceeds the boiling point, the flow rate of the combustion support gas is reduced to suppress the amount of heat discharged with the exhaust gas, thus improving the heat exchanging efficiency.
In one embodiment, a plurality of the tubes are provided for each of a plurality of rows in the path of the flow of the fuel gas, and the number of the tubes in an upstream row is greater than that of the other rows.
Upstream in the flow of the fuel gas, the object fluid has a high temperature and expands when it vaporizes, so that the number of tubes is increased to increase the total cross-sectional area of the tubes, thus limiting pressure loss.
In one embodiment, a plurality of the tubes are provided for each of a plurality of rows in the path of the flow of the fuel gas, and the surface area of the fins of the tubes in an upstream row is smaller than that of other rows.
Upstream in the flow of the fuel gas, the temperature of the object fluid is high, and the fins and tubes are prevented from being heated more than needed by reducing the surface area of the fins.